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BVDV/trich risk assessment tools

Being able to accurately estimate disease risk would be very helpful to veterinarians, but often the right tools are not available to get this done. Mike Sanderson, DVM, MS, Kansas State University, and graduate student, Becky Smith, DVM, have developed two disease-risk models, however, that may be able to help veterinarians estimate the risk of bovine viral diarrhea virus (BVDV) and trichomoniasis caused by Tritrichomonas fetus.

“BVDV seems to be a hot topic so in some cases it may get blamed for more than it does,” Sanderson notes. “Trich is a growing problem and I think veterinarians are becoming more aware of it again. Correctly estimating herd risk is a complex and difficult issue. These new models are meant to help veterinarians and producers make the most economical decisions about how to control that risk.” Both of the models can be used to identify the optimal testing and vaccination strategy for a given management plan to control the risk of introducing trich or BVDV. They might also suggest where management could be changed to decrease risk.

Trichomoniasis

Trichomoniasis, a venereal disease that can cause early abortions, has been fairly isolated in geographic pockets until a few years ago. “A few years ago no one thought trich was much of a problem, but it seems to be re-emerging and moving into areas where it hadn’t been recognized in some time,” Sanderson explains.

The Trichomoniasis page of the website asks specific questions under the categories of herd management, imports and neighboring herds. Once the information is entered, the program calculates the probability of introducing trich and the probability of exceeding a certain amount of cost associated with the outbreak.

BVDV

For BVDV specifically, the model will help identify the most economical long-run testing and vaccination control strategy including which animals to test. “Often targeted testing of imports — especially fetuses of imported pregnant females — is the best testing plan and additional testing doesn’t always pay to decrease entry risk,” Sanderson explains. “There are many scenarios where just testing the whole calf crop every year is not the most economical choice. The web model doesn’t directly address the scenario where you know you already have BVDV. In that case, you probably need to test calves to get rid of it first.”

Using the information

Sanderson and Smith spent two years developing the models and another year to get them programmed and implemented on the Web. “We did an extensive review of the scientific literature to identify all the available data on BVDV and trich,” he says. “We also surveyed some experts on areas where there was not much scientific literature. This defined the factors we wanted to model and the values that go into the model to make it work.”

He suggests the best way for a veterinarian to use these models with clients is to sit down with him/her and fill in the information as accurately as they can. “The veterinarian helping the producer input good numbers and interpret results is essential,” he notes. “I think it could also be a great way to start some discussion on risks and overall herd health.”

At the top of the results page is a probability of introducing BVDV (or trich) in your management scenario if you do nothing to prevent it, Sanderson says. That by itself is not a very useful number. “The point of the model is to identify what are the most effective and economical control strategies to minimize the economic risk of disease introduction.”

The set of numbers below show the probabilities of exceeding the reported dollar value in costs (for BVDV and a 300-cow herd they are reported for three levels: $30,000, $21,000 and $15,000) for each of the control strategies listed. “This lets the producer decide how much financial risk they are willing to take; sometimes the optimal strategy is different for different loss levels.” So for the example given here, a 300 cow herd imported 50 pregnant heifers and three bulls each year and shared a fenceline with one other herd during breeding. “This results in a 99% chance of importing BVDV over 10 years if no control is practiced,” Sanderson says. The control strategy that gives the lowest probability of exceeding $30,000 in costs is to vaccinate all breeding stock and test all imported cattle, including the calves of pregnant imports.

The model identifies the strategy or strategies with the lowest probability of exceeding the loss at each level and tags it with a “Best Choice” label. “If the model says you should vaccinate and test all imports, it’s because that strategy has the lowest probability of exceeding the designated cost,” Sanderson explains. The costs associated with each control strategy also include all costs associated with testing, labor, vaccination, etc. The model is meant to provide decision support for choosing the most economical testing and vaccination control strategy for a producer’s management.

While the models may seem complex, they effectively deal with a complex interaction of biology and economics. “Biosecurity program costs accrue each year but outbreaks happen only occasionally, so in years where no disease would be introduced, the most economical plan would be to spend no money on biosecurity” Sanderson says. “But in years when disease is introduced and an outbreak occurs, the losses can exceed many years of biosecurity program costs. The problem is how to assess the frequency of outbreaks, the cost of outbreaks, and the effectiveness of various control strategies.”

Sanderson says much data are available on BVDV and trich in the scientific literature and was utilized in the models. “For a variety of reasons we cannot directly collect the data on the interaction of all of these management and risk issues,” he says. “Imagine enrolling 200+ herds in a clinical trial and randomly imposing import practices and biosecurity practices on them — the cost would be prohibitive both in dollars and in angry ranchers. So risk analysis simulation modeling is a way to capture this as best we can and make better decisions based on what pays best or minimizes loss in the long run.”